Soy isoflavones
Summary
Soy isoflavone supplements (median 54 mg/day) cut hot flash frequency 20.6% and severity 26.2% vs placebo in a 2012 meta-analysis of 17 trials (grade B), with large differences between trials. They did not slow bone loss in Western women (no effect). A 5-year trial at 150 mg/day found more endometrial hyperplasia (3.4% vs 0%).
Key facts
- Plant estrogen-like compounds from soybeans, mainly genistein and daidzein, extracted or synthesized[1]
- Median 54 mg/day (aglycone equivalents) for 6 weeks to 12 months; 150 mg/day for 5 years in a safety trial[1][3]
- None set; isoflavones are not essential nutrients[1]
- Levothyroxine: possible interference from soy, mainly in case reports; one crossover study found none[4]
- Endometrial hyperplasia rose to 3.4% vs 0% with placebo after 5 years at 150 mg/day[3]
- 4 graded human outcomes on this page
- October 10, 2026
How it works
ProposedGenistein and daidzein bind estrogen receptors, especially the beta subtype, far more weakly than the body's own estrogen. That partial estrogen-like action is the proposed reason they ease hot flashes; supplements richer in genistein were about twice as effective in the 2012 meta-analysis.[1]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Menopause | Grade B | Improved | Meta-analysis of 13 trials · n = 1,196 · 2012 | 2 rows |
| Safety | Grade C | Worsened | RCT · n = 376 · 2004 | 1 row |
| Bone | No effect | No effect | Meta-analysis of 12 trials · n = 1,433 · 2010 | 1 row |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Hot flash frequency | Improved | Grade B | Meta-analysis · n = 1,196 · 2012 Perimenopausal and postmenopausal women in 13 RCTs Form: extracted or synthesized soy isoflavone supplements · Dose: 30–80 mg/day, median 54 mg/day (aglycone equivalents) · Duration: 6 weeks to 12 months Hot flash frequency fell 20.6% more than placebo (95% CI 12.9–28.4%). Supplements with more than 18.8 mg genistein were over twice as effective. Heterogeneity was substantial (I² 67%). | 22433977 (opens PubMed)Source [1] |
| Hot flash severity | Improved | Grade C | Meta-analysis · n = 988 · 2012 Perimenopausal and postmenopausal women in 9 RCTs Form: extracted or synthesized soy isoflavone supplements · Dose: Median 54 mg/day (aglycone equivalents) · Duration: 6 weeks to 12 months Severity fell 26.2% more than placebo (95% CI 10.2–42.2%), with very high heterogeneity (I² 86%). | 22433977 (opens PubMed)Source [1] |
| Endometrial hyperplasia (safety) | Worsened | Grade C | RCT · n = 376 · 2004 Healthy postmenopausal women with an intact uterus (Italy) Form: soy isoflavone tablets · Dose: 150 mg/day · Duration: 5 years Endometrial hyperplasia occurred in 3.37% on isoflavones vs 0% on placebo; no cancers were found. | 15237003 (opens PubMed)Source [3] |
| Lumbar spine bone mineral density | No effect | No effect | Meta-analysis · n = 1,433 · 2010 Perimenopausal and postmenopausal Western women in 12 studies Form: isoflavone mixtures (one study of isolated genistein) · Dose: Varied across studies · Duration: Varied across studies No significant effect on spine bone density; excluding the genistein study, mixtures showed no bone-sparing effect. Higher doses or longer use did not change this. | 20673147 (opens PubMed)Source [2] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Soy isoflavone extract tablets or capsules | Varies[1] | No human absorption data in our sources. Effects depend partly on genistein content.[1] | Adverse effects not detailed in the meta-analysis abstract; endometrial hyperplasia signal at 150 mg/day for 5 years.[1][3] | Hot flashes, bone density |
| Isolated or synthetic genistein | Varies[2] | No human absorption data in our sources.[2] | No form-specific tolerance data in our sources.[2] | Bone density (one study in the Western-women meta-analysis) |
| Soy foods (tofu, soy milk, edamame) | Varies[1] | Food sources; the cited trials tested supplements, not foods.[1] | Food amounts; not assessed here.[1] | Background dietary intake |
Studied doses
Doses studied in human trials ranged from about 30 to 150 mg/day of isoflavones, with a median of 54 mg/day in hot flash trials (6 weeks to 12 months) and 150 mg/day in a 5-year safety trial.[1][3]
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
- LevothyroxineReviewCase reports suggest soy may reduce levothyroxine absorption, but the one randomized crossover study found no difference; any effect appears small.[4]
Who should be careful
- Intact uterus and long-term use
- A 5-year trial at 150 mg/day found more endometrial hyperplasia (3.4% vs 0%). Long-term high-dose use with an intact uterus deserves clinician follow-up. The journal has since issued an expression of concern about that paper (2025).[3]
- Hormone-sensitive conditions
- Isoflavones act weakly on estrogen receptors. People with a history of hormone-sensitive cancer should discuss use with their oncology team; our sources do not settle this question.[1]
- Thyroid medicine
- If you use levothyroxine, keep timing consistent and recheck levels if you start or stop soy supplements.[4]
- Pregnancy and breastfeeding
- No safety data for isoflavone supplements in pregnancy or breastfeeding in our sources.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
How to read the label
- Label dose vs clinical doseHot flash trials used 30–80 mg/day (median 54 mg/day) of isoflavones measured as aglycones. Some labels list the glycoside weight, which is heavier, so the same number can mean less active isoflavone.[1]
- Look for genistein contentProducts with more than about 19 mg genistein per day were over twice as effective for hot flash frequency in the 2012 meta-analysis.[1]
- Extract weight is not isoflavone weightA label listing "soy extract 500 mg" may contain a much smaller amount of isoflavones; look for mg of total isoflavones.[1]
Cost per studied dose, without the sales pitch
Compare any product you already have by the amount it delivers, not by capsule count or front-of-pack weight.
Cost per studied dose
Enter the price, servings and the amount per serving from your label.
Frequently asked questions
Do soy isoflavones help hot flashes?
Yes, modestly. A 2012 meta-analysis of 17 trials found about 21% fewer and 26% less severe hot flashes than placebo, though results varied a lot between trials.[1]
How much soy isoflavone was used in studies?
A median of 54 mg/day (aglycone equivalents) for 6 weeks to 12 months; one safety trial used 150 mg/day for 5 years.[1][3]
Do soy isoflavones protect bones?
Not in Western women: a meta-analysis of 12 studies found no significant effect on spine bone density.[2]
Are soy isoflavones safe for the uterus?
A 5-year trial at 150 mg/day found endometrial hyperplasia in 3.4% vs 0% on placebo, with no cancers. Shorter, lower-dose use has less data on this point.[3]
Does soy interfere with levothyroxine?
Possibly a little. Case reports suggest interference, but the only randomized crossover study found no change in absorption.[4]
How long do soy isoflavones take to work for hot flashes?
Trials ran from 6 weeks to 12 months; the meta-analysis did not report a single onset time.[1]
Sources
- [1]Extracted or synthesized soybean isoflavones reduce menopausal hot flash frequency and severity: systematic review and meta-analysis of randomized controlled trials.. Menopause, 2012. meta-analysis
- [2]Soy isoflavones and bone mineral density in perimenopausal and postmenopausal Western women: a systematic review and meta-analysis of randomized controlled trials.. J Womens Health (Larchmt), 2010. meta-analysis
- [3]Endometrial effects of long-term treatment with phytoestrogens: a randomized, double-blind, placebo-controlled study.. Fertil Steril, 2004. RCT
- [4]Interference or Noninterference Between Soy and Levothyroxine: That Is the Question. A Narrative Review of Literature.. Endocr Pract, 2023. review
What changed
- First draft. Every PMID checked against NCBI E-utilities esummary (title and year taken from the record) and abstracts read for the cited rows.
- Adversarial review pass: hot flash effect sizes re-checked against the abstract; severity downgraded to C for heterogeneity; the endometrial hyperplasia signal surfaced in summary and key facts; levothyroxine wording matched to the review conclusion. Page set indexable.
- Second review: flagged the 2025 journal expression of concern on the 5-year endometrial trial (PMID 15237003) in its evidence row and the safety text.